Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 191734
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1134 AM MDT MON SEP 19 2016

.UPDATE...

Updated to include 18Z aviation discussion below.

&&

.PREV DISCUSSION... /issued 226 AM MDT MON SEP 19 2016/

SHORT TERM...Monday through Wednesday night

We still have a few showers and embedded thunder early this morning
across northern and western portions of the state as the right rear
portion of a jet streak moves away from the area. The models
indicate most of this ending by the morning though. Another weak
shortwave will pass by to the north today and there will be just
enough moisture for a couple of showers and possibly a thunderstorms
through this evening. However, since forcing will be less coverage
should be more limited. The main concern today will once again be
fire. Wind will be a bit less than yesterday. However, there will
still be some gusts over 25 mph. In addition, with drier air in
place relative humidity will be a bit lower. As a result, we have
upgraded the watches to Red Flag Warnings for southern Wyoming.

The above normal temperatures will look to peak on Tuesday as the
flow turns more to the southwest. Some areas could see high
temperatures in the upper 80s. This also looks to be the driest day
on the three as well. A weak shortwave may pass close enough to far
northwestern Wyoming to bring a spotty shower, but for now we have
left the forecast dry. Fire danger will again be a concern. Relative
humidity will be even lower. The limiting factor looks to be wind as
these look to be marginal. We will let the day shift evaluate this
and make the call on any extension of the fire weather highlights.

The pattern will be making a turn to a cooler and wetter look
starting on Wednesday as a deep trough slowly begins to move into
the area from the west. Moisture in the south to southwesterly flow
will begin to move into the area increasing clouds. For now, it
looks like areas East of the Divide should have one more dry for
Wednesday, although temperatures will be cooler with more cloud
cover. Showers and thunderstorms should spread West of the Divide in
the afternoon and continue into the evening. There is a fair amount
of QPF with the system, although with the convective nature of the
precipitation amounts will vary. Moisture will then spread East of
the Divide on Wednesday night, mainly in areas favored with south to
southwesterly flow.

LONG TERM...Thursday through Monday

Models remain in fairly decent agreement with the approaching area
of low pressure Thursday, with similar variances displayed in the
most recent round of model solutions. The area of low pressure
starts to dive into the desert southwest Thursday afternoon, with
strong south-southwest flow anticipated over much of Wyoming.
Moisture will ride along this flow to bring showers, with widespread
activity looking more favorable for areas west of the Continental
Divide.

The area of low pressure moves near the UT/ID border by Friday
afternoon, with cooling temperatures and showers starting to spread
east of the divide. Models do vary a bit with the progression of the
system late Friday evening, mainly dealing with when the system will
move through the state. The GFS is the most aggressive model with
moving the low northwest, pushing it into north-central Wyoming by
Friday evening. The ECMWF keeps the core west of the state until
Saturday morning, though has the system moving quickly through the
state, with both models indicating the low will be over south-
eastern Montana by Sunday afternoon. As the system moves into the
area, expect lowering snow levels across the state, with a rain-snow
mix possible across much of the area west of the Divide Friday
night. Valley/basin floors east of the divide are still looking to
be prevailing rain, though snowfall will be present over their
nearby mountains.

As the system lifts northeast, northwest flow will build over the
area, though some showers will continue across north-central
Wyoming. The ECMWF shows more shower activity lingering across
western Wyoming through Saturday afternoon, though this solution
seems to be a slight outlier.

Widespread dry conditions develop by Sunday afternoon, which are
expected to stay through at least Monday. Warming temperatures will
be possible under this flow.

AVIATION.../18Z ISSUANCE/

The main aviation concern will be very isolated -shra and even one
or two thunderstorms through early evening over the northwest and
extreme north. Main hazard will be gusty winds. Otherwise, VFR
conditions will prevail. Breezy to windy west to southwest wind in
most areas decreasing around sunset. On Tuesday, isolated shower
activity will be confined to the extreme northwest. Breezy south
to southwest wind will be observed over the northwest including
KJAC with weaker winds over the far south.

FIRE WEATHER...

..RED FLAG WARNING FOR SOUTHERN WYOMING THIS AFTERNOON...

A combination of low relative humidity and a gusty west to southwest
will bring critical fire conditions to southern Wyoming. Elsewhere,
elevated fire conditions are possible with low humidity and breezy
conditions in the afternoon. Isolated showers will remain possible
across portions of northern and western Wyoming. Mixing and smoke
dispersal will range from good to excellent. Warm and dry conditions
will continue into Tuesday before cooler and wetter weather moves in
for the middle and end of the week.

&&

.RIW Watches/Warnings/Advisories...
Red Flag Warning until 8 PM MDT this evening for WYZ277-279-289.

&&

$$

UPDATED...WM
SHORT TERM...Hattings
LONG TERM...AR
AVIATION...WM
FIRE WEATHER...Hattings



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